The present disclosure generally relates to subterranean treatment fluids, and, more specifically, to treatment fluids that can mitigate the occurrence or effects of precipitation in a subterranean formation by complexing a metal ion therein during a treatment operation.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. Illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations can stimulate a subterranean formation to increase production therefrom. During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, or to remove acid-soluble precipitation damage in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., proppant particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates. Other substances can be dissolved as well during the course of performing an acidizing operation. As discussed below, certain components dissolved during an acidizing operation can be problematic and possibly detrimental for future production from the subterranean formation.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the formation components. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Acidizing a siliceous formation or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the mineral and organic acids that can be effective for acidizing a carbonate formation may have little effect on a siliceous formation, since these acids do not effectively react with siliceous materials to affect their dissolution. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent, where the low pH state may promote initial silicon or aluminum dissolution and aid in maintaining these substances in a dissolved state.
Despite the advantages that can be realized by acidizing a siliceous formation, silicon and/or aluminum can produce damaging precipitation after their dissolution that can sometimes be more detrimental for production than if the acidizing operation had not been performed in the first place. Unless preventative measures are taken, some of which are discussed below, the equilibrium solubility levels of silicon and aluminum in a fluid usually depend upon one another. That is, by maintaining high levels of dissolved aluminum during an acidizing operation conducted with hydrofluoric acid, high levels of dissolved silicon can be maintained as well. In this regard, dissolved aluminum can be maintained in a fluid by coordination with fluoride ions, but such aluminum coordination can leave insufficient remaining fluoride ions for effective silicon solubilization to take place. Damaging silicon precipitation can occur as a result. Chelating agents can be used to increase the degree of silicon solubilization by maintaining aluminum in a dissolved state. Specifically, by chelating aluminum to form a soluble aluminum complex, increased levels of dissolved silicon may be realized, since more free fluoride ions are left available to affect its solubilization. In addition to chelating agents that target a metal ion, particularly aluminum, other types of complexing agents can be employed that directly complex silicon and promote its solubilization.
Iron dissolution can also be problematic during acidizing operations, particularly dissolution of ferrous iron. Ferric iron can form in the presence of dissolved oxygen and can later precipitate as ferric hydroxide above a pH of about 2. Ferric hydroxide precipitation represents an operational concern due to its highly gelatinous consistency. Ferric iron can also result from tubing pickling. The latter can usually be effectively managed by flowing out the fluid containing the iron dissolution products, although this added step may add to process complexity and cost. Due to the damage potential represented by ferric iron, its chelation may also be desirable while downhole.
Even when chelating agents are used, precipitation of insoluble fluorosilicates and aluminosilicates can sometimes be problematic in the presence of Group 1 metal ions (i.e., alkali metal ions). The terms “Group 1 metal ions” and “alkali metal ions” will be used synonymously herein. Under low pH conditions (e.g., below a pH of about 3), dissolved silicon can react with Group 1 metal ions (e.g., Na+ and K+) to produce insoluble alkali metal fluorosilicates and aluminosilicates. Other metal ions, including Group 2 metal ions (e.g., Ca2+ and Mg2+), may also be problematic in this regard. In many instances, costly pre-flush fluids may be introduced to a subterranean formation prior to performing an acidizing operation therein in order to decrease the quantity of available alkali metal ions. In some instances, such pre-flush fluids can contain ammonium ions (NH4+) that can displace alkali metal ions in the subterranean formation and leave it desirably conditioned for acidization. In contrast to alkali metal ions, ammonium ions are not believed to promote the formation of insoluble fluorosilicates and aluminosilicates. The use of pre-flush fluids, particularly those containing ammonium ions, can considerably add to the time and expense needed to perform an acidizing operation. In addition to problematic alkali metal ions in the subterranean formation, the precipitation of alkali metal fluorosilicates and fluoroaluminates can considerably limit the sourcing of carrier fluids that may be used when acidizing a subterranean formation.